Lacrimal gland

Conjuctival goblet cells

Contains salts, proteins and lyzozyme, Transports substances to and from the cornea, Prevents infection, Maintains the tonicity of the tear film

Provides a hydrophillic suface to allow an even distribution of tear film.

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What does the nasolacrimal system involve?

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What can lead to dry eye?

What are the symptoms?

An abnormality in any of the structures involved in the production and secretion of the various tear layers can lead to dry eyes.

It can be mild, causing intermittent blurring, foreign body sensation or reflexive tearing or it can be severe with significant damage to the eye. An abnormality in any of the structures involved in the production and secretion of the various tear layers can lead to dry eyes.

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What is the cornea?

What are the 5 layers?

What are the functions?

Transparent AVASCULAR tissue

Epithelium

Bowman's Membrane

Stroma

Descemet's Membrane

Endothelium

Transparency- Relative dehydration of the stroma is maintained by the impermeable epithelial barrier and active pumping mechanisms of the corneal endothelium. The regular spacing of individual stromal collagen fibrils.

Refraction - The cornea is the major refractive component of the eye - 48/58 dioptres

Barrier to infection and trauma

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What is the crystalline lens?

What are the 4 layers?

Which layer contains the more recently formed fibres?

A biconvex structure with a clear outer acellular capsule. Has a relatively low water (66%) and high protein (35%) content. Consists of very tightly packed hexagonal fibres

Embryonic Nucleus, Fetal Nucleus, Adult Nucleus, Cortex.

The cortex. The nucleus contains the older non-dividing cells.

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How is the crystalline lens transparent?

What is a cataract?

Orderly arrangement of lens fibres. Small difference in refractive index between the various components. Absence of blood vessels

What is the optic density of a medium called?

What is the refractive index for air?

eye

water

Explain refraction

What does amount of refraction depend upon?

The refractive index (m)

m= 1.0

m for lens of the eye =1.4

m for water =1.33

A ray of light passing through an interface where the refractive index changes is refracted (changes direction)

Amount of refraction depends on incident angle

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How is the image inverted in the eye?

Both vertically and laterally

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Outline Accomodation

What causes the ciliary muscle to contract?

When is this useful?

What occurs as a result?

What happens when the ciliary muscle relaxes?

When is this useful?

Lens can be deformed (by ciliary muscle action and elasticity of the lens)

Parasympathetic nerves cause ciliary muscle to contract

When looking at a near object

reduces the tension in the ligaments, lens allowed to assume rounded shape

Lens stretched into flattened shape,suspensory ligaments tense and pull outwards on the lens

When looking at a distant object

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What are some limitations of the mechanism of accomodation?

What is the focusing power of the lens measured in?

How can it be calculated?

1.Depends on elastic fibres in lens tissue

2.Reduces with age

3.Loss of accommodation with age (presbyopia)

Dioptres

D = 1 / focal length in metres

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What is the solution for presbyopia?

A corrective CONVEX lens

this augments the focusing power to bring the retinal image to a focus on the retina.

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What is Emmetropia?

Rays are focused

No correction required.

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What is Myopia?

How is it corrected?

Rays focus in front of retina = "short-sighted”

Correct with negative CONCAVE lens

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What is Hypermetropia?

How is it corrected?

Rays focused behind retina- ”long-sighted”

Correct with positive CONVEX lens

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Amblyopia

What is it?

Why might the brain not recieve a clear image?

Why can there be permanent damage?

When can it be corrected?

Occurs when one eye experiences a blurred view and the other a normal view, but the brain only processes the normal view

•one eye has a larger refractive error than the other

•Eyes are misaligned (strabismus or “squint”)

•The optical path is obstructed (congenital cataract)

In these cases the neural connections associated with the eye with the blurred vision will not form correctly and therefore the eye will have worse vision forever (ambylopia or “lazy eye”) even if the problem is corrected in later life.

The first 7/8 years are termed “plastic period” in ocular development as this is when the neural connections are formed. So if the problem is corrected within this period and the eye is forced to focus through the use of a patch, visual acuity can be improved.

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Iris

what are the two optical functions of the iris?

How is the light entering the eye modulated?

Where does this control originate from?

Which muscles are under PARASYMPATHETIC CONTROL? What do these do?

Which muscles are under SYMPATHETIC CONTROL?

- restrict amount of light entering eye

- change depth of focus (range over which objects are in focus).

by changing the size of the pupil

The brain stem

The circular muscles, reduce pupil size

Radial muscle, increase pupil size

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The Humors

Where is the VITREOUS HUMOR? Give it's characteristics.

Where is the AQUEOUS HUMOR? Give it's characteristics.

In the anterior chamber. High viscosity, static.

In front of the lens. Low viscosity, high turnover rate.

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The Humors

What is the function of the Aqueous Humor?

What is the function of the Vitreus Humor?

Structure: Maintains the intraocular pressure and inflates the globe of the eye.

flow of aqueous humour helps maintain intraocular pressure (IOP), necessary for the proper shape and optical properties of the globe.

It nourishes the cornea and lens as well as the trabecular meshwork (TM).

It provides a transparent and colourless medium of refractive index 1.33332 between the cornea and lens, thus constituting an important component of the eye's optical system.

behind the iris by the ciliary body.

It flows through the pupil and drains away at the angle between the cornea and iris (the drainage angle).

1. From within the drainage angle, the aqueous humour passes through a porous tissue known as the trabecular meshwork – into a collector channel (Schlemm’s canal), which empties into veins under the conjunctiva and back into the bloodstream.

Aqueous humour can freely pass between the ciliary muscle bundles into the supraciliary and suprachoroidal spaces, from which it is drained through the sclera as there is no epithelial barrier between the anterior chamber and the ciliary muscle,

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What is intraocular pressure maintained at?

What is the balance between?

10-21 mm Hg

secretion and drainage of aqueous humour

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What is Glaucoma?

What can some types cause?

How is it diagnosed?

A group of eye disorders with progressive damage to the optic nerve- loss of nerve fibres = visual field defects.

Where is the vitreous normally attached?

What are flashes and floaters? What can they be symptoms of?

What is the Vitreous humour made up of?

1.Around the anterior border of the retina

2.At the fovea

3.At the optic disc

With age the vitreous gel changes its consistency and clumps can form which appear in the patients visual field.

Retinal or vitreous detactchment

. It is produced by certain retinal cells. It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the hyaluronic acid), no blood vessels, and 98-99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibers with the glycosaminoglycan hyaluronic acid, and also a wide array of proteins in micro amounts.

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What is binocularity?

What are the stipulations for it?

-A pair of eyes whose function as a unit is greater than the sum of the function of each individual eye allowing a greater visual field and, by overlapping the fields, perception of depth by stereopsis (3D vision)

•Both eyes must act in concert

•Good vision is required from each eye

•The line of sight of each eye must at all times be pointing to the same visual target

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What mechanisms allow the eye to move and work as a single functional unit?

What supplies the external part of the retina?

What supplies the internal part?

The choriocapillaris (from the short posterior ciliarty arteries

branches of the central retinal artery, (which do not anastomose with each other)

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What does the retina have such a rich blood flow?

It has a high metabolic rate

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Where is there NO retinal blood supply?

the MACULA

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Descibe the blood flow in the retina

Capillaries inner

Choroid outer

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Retina

What is it's function?

How many layers does it have?

1. Transparent

Converts light energy into nervous impulses - transduction

Has Photoreceptors = 120 million rods (monochromatic) and 6 million cones (colour vision)

2. 10 layers

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How is visual information transmitted in the retina?

from photoreceptors to bipolar neurons and ganglion neurons before exiting the eye via the optic nerve.

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From inner to outer, name the cells in the retina

Inner limiting membrane

nerve fibre layer

ganglion cells

amacrine cells

biopolar cells

horizontal cells

muller cells

outer limiting membrane

rods and cones

pigment epithelium

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Where is visual acuity best?

Central retinal region (fovea)

(light doesnt pass through sensory neurons before striking the photoreceptors.)

CONES ONLY

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Photoreceptors - Rods and Cones

How do they differ?

1.Shape

2.Range of operation

3.Distribution

4.Connectivity

5.Visual function

6.Photopigment

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Describe the macular pigment

•Found in mainly Henle fibre layer.

•Absorbs and attenuates blue light.

•Reduces Chromatic abberation.

•Photoprotective?

Concentrated yellow pigment – mainly within the nerve fibre layer – can’t make this pigment, completely dietary in origin. Make up of antioxidants. May protect this part of the eye form oxidative stress. May sharpen acuity at this part. Research ongoing

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In what part of the rods and cones does phototransduction take place?

In outer segments

(visual pigments in membrane discs)

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How do rods and cones differ in terms of range of operation?

Rods = black and white vision (498nm)

Cones = colour vision (420nm - blue, 534nm - green, 564nm - red)

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How do rods and cones differ in terms of distribution?

Rods everywhere expect fovea

Cones highly concentrated in the fovea

NEITHER at the optic disk

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How do rods and cones differ in terms of connectivity?

Many rods are likely to synapse onto one ganglion cell - allowing for HIGH SPATIAL SUMMATION (– pull together the light energy falling on a wide area of the retina) Nb. these usually work in low light.

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How do rods and cones differ in terms of visual function?

Rods:

•High spatial summation

•High scotopic sensitivity

•Poor resolution

Cones:

•Low spatial summation

•Poor scotopic sensitivity

•High resolution

Scotopic vision is the vision of the eye under low light conditions

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How do rods and cones differ in terms of photopigment?

Rods: Rhodopsin (peak 496nm)

Cones: all contain 11-cis retinal + opsin

3 different photopigments

-Short wavelength sensitive (peak 420 nm) BLUE

-Medium wavelength sensitive (peak 530nm) GREEN

-Long wavelength sensitive (peak 560nm) RED

Notice that S-cones are rare, and even absent in a small part at the very center.

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Compare rods and cones

Rods

Achromatic

Peripheral retina

High convergence

High light sensitivity

Low visual acuity

Cones

Chromatic

Central retina (fovea)

Low convergence

Low light sensitivity

High visual acuity

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What is the

L cone pigment?

M cone pigment?

S cone pigment?

What chromosomes are these pigment genes located on?

Protan (red)

Deutan (green)

Tritan (blue)

Protan and Deutan = X-chromosome, showing high variability in the normal population. Tritan encoding gene is on Chr 7

+ suffix “opia” = cone is absent

+ suffix “omaly”= abnormal cone function

Protan + deutan are referred to as red-green anomalies

More MEN have inherited colour vision deficiencies.

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How are Protanopia and Deutanopia inhertied?

as X-linked recessive traits

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What is dark adaptation?

What is recovery related to?

Process of restoration of retinal sensitivity after exposure to bright illumination. Rods and cones recover visual sensitivity at different rates.

cones conduct CA2+ at a faster rate than those in rods (Frings et al, 1995), and Ca2+

efflux in cones is five to eight times faster than that seen in rods

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What is AMD?

What is the pathophysiology?

Which photoreceptors are affected more in the early stages?

Age-related macular degeneration

Accumulation of waste material known as drusen in Bruch’s membrane and RPE leads to impaired dark adaptation.

In the early stages rods are affected more than cones therefore dark adaptation and dark adapted sensitivity measurements are able to pick up deficits much earlier than standard measures such as visual acuity.

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Outline Transduction in photoreceptors

Light entering the eye activates the opsin molecules in the photoreceptors

Activated rhodopsin causes a reduction in the cGMP intracellular concentration

cytoplasmic cGMP levels are controlled by cGMP phosphodiesterase (in the dark, this enzyme is largely inactive)

photoreceptor is hyperpolarized following exposure to light

In the dark, cGMP levels in the outer segment are high; this molecule binds to the Na+-permeable channels in the membrane, keeping them open and allowing sodium (and other cations) to enter, thus depolarizing the cell. Exposure to light leads to a decrease in cGMP levels, a closing of the channels, and receptor hyperpolarization.

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Explain phototransduction in rod receptors

. (A) The molecular structure of rhodopsin, the pigment in rods. (B) The second messenger cascade of phototransduction. Light stimulation of rhodopsin in the receptor disks leads to the activation of a G-protein (transducin), which in turn activates a phosphodiesterase (PDE). The phosphodiesterase hydrolyzes cGMP, reducing its concentration in the outer segment and leading to the closure of sodium channels in the outer segment membrane.

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Explain the termination of the phototransduction cascade

(1) Inactivation of rhodopsin occurs through phosphorylation by the opsin kinase, followed by the binding of arrestin to phosphorylated rhodopsin.

(2) Inactivation of transducin occurs through the hydrolysis of bound GTP to GDP (Tα-GTP to Tα-GDP) via an intrinsic GTPase activity that is accelerated by the GTPase activating protein RGS9 (regulator of G-protein signaling).

(3) Inactivation of phosphodiesterase (PDE) is coupled to the inactivation of transducin. Inactivated transducin (Tα-GDP) dissociates from PDE, resulting in a cessation of PDE-mediated cGMP hydrolysis.

Uses low-coherence interferometry to produce 2D images of optical scattering from integral tissue microstructures in a way that is analagous to ultrasonic pulse-echo imaging.

Hyperreflectivity: Increase in reflectivity.

Hyporeflectivity: Decrease in reflectivity.

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In-vivo RD-OCT versus histology

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International Nomenclature

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What is the optic disc?

formed by axons from ganglion cells in the retina

These axons pass through the lamina cribrosa and form a cable behind the globe that will travel through the orbit, optic canal, and decussate in the chiasm and synapse to the next neuron in the lateral geniculate.

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Describe evaluation of the optic disc

What will give a larger cup to disc ratio?

a smaller cup to disc ratio?

The optic nerve is divided into tenths and the cup is compared to the entire optic nerve (optic disc) to obtain the cup-to-disc ratio.

shallow nerve fibre layer

thick nerve-fibre layer (nearly a non-existant cup)

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What is the optic disc?

•The entry of the optic nerve into the eye (1.5 x 1.5 mm)

•Corresponds to the blind spot of the visual field as does not contain any overlying photoreceptors

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What is the optic nerve?

•Contains over 1 million fibres

•Nerve fibres are myelinated only after leaving the eye

•Nasal fibres decussate at the optic chiasm

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What is the optic tract?

•Extends from the optic chiasm to the LGN

•Contains uncrossed and crossed fibres from the temporal and nasal retinas respectively

•About 30% of the fibres leave the visual pathway in the optic tract before the LGN, including those subserving the pupillary light reflex which pass to the pretectal nuclei

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What are the optic radiations?

•Axons of the third neuron of the visual pathway originate in the LGN

•The anteroinferior fan out as Meyer’s loop by passing down into the temporal lobe

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Where does the optic nerve terminate?

Describe the next projections

on the cells of the lateral geniculate nucleus (first relay in the brains visual pathway)

The image captured by each eye is transmitted to the brain by the optic nerve.

The cells of the lateral geniculate nucleus then project to their main target, the primary visual cortex. It is in the primary visual cortex that the brain begins to reconstitute the image from the receptive fields of the cells of the retina.

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What names are given to the primary visual cortex?

where is it?

Striate cortex, or simply V1

located in the most posterior portion of the brain's occipital lobe.

(corresponds to Area 17 described by the anatomist Brodmann in the early 20th century)